The present invention relates to metal seals for sealing various wellhead devices to casing and tubing in oil and gas wells.
Oil and gas wells typically include one or more pipe strings extending downwardly into the earth from its surface. The strings are included one within the other and serve various purposes, such as maintaining the structural integrity of the well and for controlling fluid flow and fluid pressures within the well. For the purposes of this application, a string is referred to as casing if there is at least one string within that string, and the innermost string is referred to as tubing.
At the wellhead, various different types of wellhead members are connected and sealed to the casing and tubing and perform various functions, among which are to support the casing and tubing from the surface, to provide means for connecting fluid conduits to the tubing as well as to the annuli defined by the tubing and the various casing strings surrounding it, and for maintaining control of the fluid pressures experienced within the wellhead. To maintain control of the often very high fluid pressures, it is necessary to provide seals between the various wellhead members and the tubing and casing. Elastomeric seals have been provided in such devices which provide a seal against the tubing or casing when the seal is pressed inwardly thereagainst. This is achieved in various devices by exerting pressure vertically against the seal causing it to expand inwardly against the tubing or casing thus to seal it off. The elastomeric seal may be urged inwardly also by pressure exerted upon its outer circumferential surface. For example, such seals have been in use for many years wherein fluid pressure is exerted in an annulus surrounding the outside diameter of the elastomeric seal thus to urge it inwardly. The annulus is connected to the exterior of the device by means of a check valve through which fluid under pressure is introduced. In some such sealing methods, a liquid plastic under pressure is injected through the check valve for forming the seal; thereafter the plastic hardens so that the seal is permanently maintained.
Many well operators believe elastomeric seals to be unreliable under extremes of temperature which may cause them to break down, leading to an undesireable breakage of the seal. Accordingly, metal type seals have been adopted for use, since they do not share the temperature sensitivity problems of elastomeric materials. Such metal seals are provided as smooth rings adapted to be pressed against the outer circumferential surface of tubing or casing to effect a seal. It is, therefore, necessary that the outer surface of the casing or tubing where the seal is to be made must be smooth and be dimensioned within very close tolerances. It is thus necessary to prepare the tubing or casing at the well site carefully by machining it to mate closely with the smooth metal seal. This procedure is both time consuming and expensive.